jQuery offset().top not working properly

jQuery offset().top not working properly

I am trying to get a div to stick once it is scrolled out of view.
var jQ = jQuery.noConflict();
jQ(document).ready(function() {
var win = jQ(window);
var navTop = jQ('#navbar').offset().top;
win.scroll(function() {
jQ('#navbar').toggleClass('sticky', win.scrollTop() > navTop);
});
});
The problem is that with this code, navTop is not calculated correctly. If
I calculate navTop in the scroll function it works as expected but with a
horrible flickering effect which I assume is due to recalculating the
value many times.
Why does it not calculate the value correctly after document is loaded?